1. Field Of The Invention
This invention relates to computer systems and, more particularly, to methods and apparatus for providing output signals to control computer monitors which require zero setup blanking and computer monitors which require pedestal setup blanking.
2. History Of The Prior Art
Typically, a computer monitor receives information from a computer system and displays that information in a picture made up of a number of horizontal scan lines on the screen of the monitor. In general, computer monitors accept the information to be displayed as an analog signal which varies from a more negative value to a more positive value. With a grayscale monitor the more negative value represents black while the more positive value represents white. The analog signal is used to modulate the raster beam of the monitor as it scans across the face of the monitor.
Typically, the scan of the raster beam starts at the upper left-hand corner of the monitor and traces a first horizontal line across the display from left to right. The data to be presented on this first line is transferred to the monitor during a period beginning after the receipt of a synchronizing signal and continuing until the line is completed.
When the scan line has been completed, the raster beam must retrace over the monitor screen to a position at which the next line starts. During this retrace interval, the raster beam must not be visible; or it will confuse the display of information on the screen. To make the beam invisible, the value of the analog signal fed to the monitor during the retrace period is typically set at a blank level more negative than the value used to indicate black. Although one would imagine that the level for black would be sufficient to keep the retrace from appearing on the monitor screen, this was not true as a historical fact. When the first television sets were produced using vacuum tubes, it was very difficult to obtain the precise levels necessary for driving the television monitors; a "black" retrace level would often show on the display. Consequently, a blank level lower than the black level was used and continues to be used for both television and computer monitors to assure that the retrace does not appear on the display.
More modern monitor systems than those typically used in the United States, such as European television systems, were designed much later in time and do not require that a more negative setup level be used during the retrace period to accomplish blanking. The blank level is simply set equal to the black level which can be closely controlled with modern components.
It appears to be clear that the United States television and computer industries will soon be using monitors in which the blank level is set to the black level, so called "zero-setup" monitors. The reasons for such a conclusion are economic. In order to provide blanking at a pedestal level more negative than the color black which the video circuit of a monitor expects to be at ground, it is necessary for the digital-to-analog converter circuitry of the computer system to include circuitry to generate the pedestal level between blank and black to boost the black level above ground. Then, it is necessary to provide additional circuitry at the monitor to provide an equal level which may be subtracted from the signal provided as input to the monitor so that an accurate black at ground level may be produced. Both items of circuitry have a cost.
Moreover, although the circuitry in the monitor may be made to produce a level very accurately, a current generator is typically used in the computer system which is usually an integrated circuit. It is very difficult to accurately control the value of the current generated by such circuitry. In fact, the current values generated by such circuitry may vary by fifty percent from the desired values. The effect of this variance is that the pedestal level for blanking from which a constant value is subtracted at the monitor may cause the black level to be too negative and clip the signal making all the dark grays become black or too positive so that a gray signal is provided for the black level making black become a dark gray. Thus, the present arrangement provides costly circuitry which actually makes the system operate less accurately than it would without the circuitry.
Although it is clear that the industry is headed toward zero setup level monitors, it is not yet there. Consequently, it would be useful during the period until such monitors become the standard that a computer system be capable of operating using presently available circuitry with both those monitors providing a pedestal level for blanking and those providing a zero setup level. This requires that the digital-to-analog converter circuitry at the output of the computer display system be capable of furnishing either data signals dropping from a pedestal level to a blanking level during horizontal retrace or data signals assuming a black level during horizontal retrace. It would also be desirable to attain such a result with minimal changes to the digital-to-analog converter circuitry presently available.